There is known a method of generating a large amount of fine gas bubbles in a liquid in order to efficiently dissolve gas, such as air, in a liquid, such as water. By generating gas bubbles with diameters of 10-50 μm in the liquid, rising speeds of the gas bubbles due to buoyancy are greatly slowed down. Therefore, the gas bubbles remain in the liquid for a longer time and the gas is dissolved in the liquid with higher efficiency.
Patent Document 1 discloses a liquid-gas agitating and mixing apparatus comprising an outer shell member having a linear cylindrical shape, and a drive member having a linear columnar shape, inserted coaxially within the outer shell member and rotated at a high speed, wherein a gap between the outer shell member and the drive member is set to the smallest possible value within a range allowing the liquid to enter the gap when the drive member is rotated at the high speed. More specifically, the liquid and the gas are caused to enter the gap between the outer shell member and the drive member, and they are agitated and mixed with each other due to vigorous vortex flow motions of the liquid, which are generated by the high-speed rotation of the drive member. The liquid containing a large number of fine gas bubbles generated with the agitation and the mixing is powerfully released through an opening at the lower end of the outer shell member, thus enabling a large number of very fine gas bubbles to float in the liquid for a long period.
In the apparatus disclosed in Patent Document 1, the peripheral speed at an outer peripheral surface of the drive member is required to be set to about 12 m/sec, and the drive member has to be rotated at such a high speed. Also, because of the necessity of agitating and mixing the liquid and the gas for a period of not shorter than a certain time, the outer shell member and the drive member must have a length of not shorter than a certain value, and they are required to have high dimensional accuracy to prevent vibration of the drive member rotating at the high speed.
Patent Documents 2 and 3 each disclose a liquid-gas agitating and mixing apparatus comprising an outer tube having a linear cylindrical shape, a rotating shaft coaxially inserted within the outer tube and rotated at a high speed, and an agitation bladed wheel in combination of a forward blade and a reverse blade which are fixed to the rotating shaft at a certain interval in the axial direction. The rotating shaft is rotated with a liquid filled in the outer tube, and gas is sucked along the rotating shaft by the sucking action due to vortex flows of the liquid. The operation of agitating and mixing the liquid and the gas is achieved with a vigorous cutting operation applied to a mixture of the liquid and the gas by individual blade pieces of the agitation bladed wheel, as well as with a mingling operation provided by collision between flow motions in the forward direction given by the forward blade and flow motions in the reverse direction given by the reverse blade.
In the apparatus disclosed in Patent Documents 2 and 3, the agitation and the mixing between the liquid and the gas are achieved with not only vortex flow motions caused by the rotating operation of the rotating shaft, but also the cutting operation of the agitation bladed wheel mounted to the rotating shaft and the collision operation between forward bubble vortex flows and reverse bubble vortex flows. Therefore, subdivision of gas bubbles can be realized in a powerful and efficient manner, whereby sufficiently subdivided gas bubbles can be obtained. As compared with the apparatus disclosed in Patent Document 1, a level of rotational speed is lower and a total weight of the rotated portion can be sufficiently reduced, whereby a level of dimensional accuracy required in forming the parts is not so very high.
The liquid-gas agitating and mixing apparatus disclosed in Patent Documents 2 and 3 is manufactured on a commercial basis and is able to generate fine gas bubbles having diameters of 10-50 μm in the liquid. As a result, the gas can be efficiently dissolved in the liquid.
In a swirling fine gas-bubble generation apparatus disclosed in Patent Document 4, a conical space is formed in a container constituting the apparatus, and a swirl flow is generated in the space by supplying a liquid under pressure in the direction tangential to an inner peripheral surface defined by an inner wall of the space. On the other hand, gas is sucked through a gas inlet formed at the bottom of the conical space in its central portion and passes along a space axis at which pressure is lowest, whereby a thin swirling gas cavity is generated. The cross-sectional area of the space is gradually reduced and the speed of the swirl flow is increased as the swirl flow advances from the inlet to an outlet. The gas continuously flows in the form of a string toward the outlet. At the same time as when the gas is discharged through the outlet, the swirl motion is abruptly weakened by the surrounding static liquid, and the string-like air cavity is continuously cut with stability. As a result, a large amount of fine gas bubbles having diameters of 10-20 μm, for example, are generated near the outlet and are released to the liquid outside the container.
Non-Patent Document 1 describes the result of measuring the number of generated gas bubbles by using a gas bubble generation apparatus which operates based on the same principle as that of the apparatus disclosed in Patent Document 4. In the gas bubble generation apparatus, water supplied to a container by a pump rises along a wall of the container, and after striking against the ceiling, the water flows toward an outlet port at a lower level along the center of a vortex flow. Gas is automatically sucked through a gas inlet due to negative pressure generated by the swirling water flow, and a gas column formed along a swirl axis is forcibly released through the outlet port together with the swirling water flow, thus generating fine gas bubbles. The capacity of a water tank is 35 liters. 1%-TFH (tetrahydrofuran) is added, as hydrate generating catalyst, to distilled water in the tank. A bubble diameter distribution is continuously measured by an optical particle distribution meter for water (LiQuilaz-E20 made in USA). The measurement is based on an optical-dynamic scattering measurement method and is performed over the range of 2 μm-125 m in terms of bubble diameter. Looking at FIG. 2 in Non-Patent Document 1, the number of gas bubbles in the liquid is measured at a pitch of 5 μm of the bubble diameter. The number of gas bubbles is maximized near the bubble diameter of 40 μm, and the gas bubbles are generated at a density of about 60 bubbles/mL within the bubble diameter range of 5 μm. On the other hand, in the zone where the bubble diameter is less than 15 μm, the gas bubbles are generated at a density of about 20 bubbles/mL within the bubble diameter range of 5 μm. It is reported that, as compared with the case using, as the liquid, distilled water containing no additives, the number of the generated fine gas bubbles is increased in the case using distilled water added with TFH or other similar material.                Patent Document 1: Japanese Examined Patent Application Publication No. 61-36448        Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-220364        Patent Document 3: Japanese Unexamined Patent Application Publication No. 6-91146        Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-447        Non-Patent Document 1: “Effect of Shrinking Microbubble on Gas Hydrate Formation”, The Journal of Physical Chemistry, Vol. 107, No. 10, 2003, pp 2171-2173        